1. Field of the Invention
This invention relates to a flux-cored wire for use in gas-shielded arc welding, and more particularly to a flux-cored wire for automatic or full-automatic welding which meets all requirements for weldability, bead contour and joint strength.
2. Description of the Prior Art
Wires for use in gas shielded arc welding and especially for carbon dioxide shielded arc welding have become more and more prevalent, which satisfy a recent demand for highly efficient and inexpensive welding. Typical prior art wires are disclosed in U.S. Pat. Nos. 3,424,892 to Wayne L. Wilcox and 3,097,979 to Francis E. Amsel et al. Disadvantages of those wires, however, are poor bead appearance, unsatisfactory fatigue strength due to a convex bead shape, increased spatter loss, etc. Hand welding rods should be used under the siutation where these disadvantageous are of serious importance. The MIG and MAG welding techniques using an inert gas such as Ar or He or a mixture of such an inert gas and CO.sub.2 and welding techniques using flux-cored wires have been proposed in the art of welding, aiming at improvements in bead contour and weldability. Such an inert gas is too expensive with too high a resulting welding cost for conventional welding.
While a considerable amount of effort has been devoted to improve the performance of flux-cored wires, these flux-cored wires still have the following shortcomings. As a rule, the flux-cored wires are classified into the basic type and acidic type according to the basicity of slags. The basic type generally includes a great deal of calcium fluoride and a low content of diffusible hydrogen, e.g. 1-3 mg/100 g, with the former being contributive to excellent physical properties of metal deposits and the latter contributive to excellent resistance to weld cracks. However, similar hand welding rods containing fluxes of the basic type shows a tendency for the slag to fall in drops and the bead to become convex with an increased amount of spattering and fuming. The acidic type, on the other hand, advantageously contains titania as its major ingredient so that the resulting bead assumes a desirable shape and appearance with a minimum of spattering. However, since the metal deposit normally contains a relatively large amount of diffusible hydrogen on the order of 8-15 ml/100 g, resistance to welding cracks is poor and pitting and porosity such as blowholes easily develop especially in welding applications to mill scales and heavily rusted steel sheets.
In case of low hydrogen welding rods, the welding rods are baked or otherwise treated for drying in the couse of manufacturing of the rods or immediately before its use in order to lower the content of diffusible hydrogen in the metal deposit. The flux-cored wires are similarly subject to such reforming treatment. However, baking demands a large expenditure of labor and fuel costs and impairs rust resistance of the wires. A special package or a rustproof painting is required as a measure of preventing rust. Conversely, if the hydrogen content of the wires is suppressed below 10-15 ppm, then baking or other thermal treatments will become necessary.